The present invention relates to an apparatus for continuously pressure-feeding slurry such as bauxite or nickel ore slurry, and more particularly, a slurry pressure feeding apparatus for preventing an undesirable closing of a change-over valve interposed in a slurry supply or discharge pipe connected to a lower portion of a supply chamber when the slurry is fed under pressure.
For example, in U.S. Pat. No. 3,556,682, there has been proposed a slurry pressure feeding apparatus comprising a plurality of supply chambers which are arranged in parallel to each other, a low pressure slurry pump for filling each of the supply chambers with slurry consisting of water and solid material, and a high pressure driving liquid pump for discharging the slurry from the supply chambers to a transferring pipe, wherein a position of a float floating on an interface between the slurry and driving liquid within each of the supply chambers is detected by an outside sensor and in accordance with the detecting signal thereof, change-over valves interposed in the supply or discharge pipe for the driving liquid and in the supply or discharge pipe for the slurry are opened/closed selectively, so that the slurry is continuously transferred under the liquid pressure.
The above described change-over valve interposed in the slurry supply or discharge pipe is of such type that the fluid passage is rendered to be communicated or interrupted by slidingly moving a valve body up and down along its valve seat under the liquid pressure of the slurry, that is, a so-called lift type check valve.
On the other hand, the above-described high pressure driving liquid pump is of the centrifugal type. Typical characteristic curves of the driving liquid pump are shown in FIG. 1, in which reference character Q indicates the output flow rate, P the pump outlet pressure, P.sub.1 the pump outlet pressure at a normal flow rate, P.sub.2 the pump outlet pressure in a pump closing condition (flow rate of zero), .DELTA.P.sub.2 the differential pressure between the closing pump outlet pressure P.sub.2 and the normal flow rate pump outlet pressure P.sub.1, Hf the pressure loss of the overall passage from the pump, and Kw the necessary power for the pump.
In the case where the differential pressure .DELTA.P.sub.2 of the driving liquid pump having the characteristic curves shown is less than a necessary pressure due to the fact that the contact area between the valve body and seat of the check valve as a whole is kept under a packing condition, an upward movement of the valve body is prevented so that the check valve is not opened. As a result, the flow of the slurry is discontinued to cause a variation in pressure, or otherwise the driving of the slurry continuous pressure feeding apparatus is impossible.
In other words, in the above described lift type check valve, when particles of slurry to be treated have a fine diameter, the overall contact area between the valve body and seat becomes an intimate surface contact to be kept under the packing condition.
The pump characteristics obtained by a rotational speed control through a fluid coupling and the like to save energy are shown in FIG. 2. Namely, if the flow rate is decreased to about 30%, the flow rate-pressure characteristic is represented by P'. As is apparent from the curve P', in the case where the flow rate is decreased to 30% through the rotational speed control, although a pump in which the differential pressure .DELTA.P.sub.2 at the flow rate of 100% is greater than packing pressure .DELTA.P.sub.1 (mention later) is selected, the differential pressure .DELTA.P.sub.2 becomes .DELTA.P.sub.2 '. Since the differential pressure .DELTA.P.sub.2 ' is equal to only several tenths of the differential pressure .DELTA.P.sub.2, under this condition, the check valve cannot be opened in the packing state in the same way as in the foregoing example.